A New Occurrence of Olivine Diabase. 
207 
Unfortunately the specimens were lost before sections had been made 
from them so that the comparison was not carried to completion. 
Perhaps the most prominent field characteristic of this rock is the 
profound decomposition it has suffered. How much of it has been re¬ 
moved by erosion it is impossible to say, but the whole exposure from 
its upper surface down to the bed of the stream, a distance of twenty to 
twenty-five feet, seems to be thoroughly disintegrated. It apparently 
maintains its vertical position only by the support of a net-work of thin 
quartz veins which ramify through it in all directions. 
The decomposition has brought out a set of lines indicating a horizon¬ 
tal movement in the fluid mass. Parallel with these apparent flowage 
lines, a layer of bowlders of decomposition, well rounded, but still in 
situ , extends for some distance along the bank of the stream. The rock 
below this layer of bowlders is as much decomposed as the upper por¬ 
tion. The limit of decomposition seems to be marked by the position of 
the stream. The rock in its bed is firm and apparently unaltered. 
Just below the lower end of the diabase exposure, an excavation was 
made to determine if possible the relations of the diabase to a bed of 
siliceous flour which occurs there. (Another bed of this material occurs 
about a mile farther down the creek resting on the quartzite.) 
About two feet below the surface a layer — not continuous — of steatite 
about two inches thick was encountered. Immediately below this 
steatitic layer was a stratum of the same material containing hard frag¬ 
ments up to a quarter of an inch in diameter. Whether these were frag¬ 
ments of sound diabase, or pieces of the quartz veins was not determined. 
Below this the rock became gradually more firm so that at a depth of 
six inches a pick could be driven into it with difficulty. 
It does not appear that the extensive decomposition of this rock is due 
to any inherent tendency in that direction, but rather to the peculiar 
circumstances of its history. 
In the warmer and moister portion of our country south of the glaci¬ 
ated area — in fact, in any ucglaciated region where ancient crystalline 
rocks occur, extensive and profound decomposition very commonly 
occurs. 
This rock, however, is in the more arid portion of the glaciated area. 
The surface of the adjacent quartzite is planed and scratched after the 
orthodox glacial fashion. 
Within half a mile of the diabase outcrop, on a tabular surface of 
quartzite, two distinct sets of strise occur. One set runs S. 20° E. and 
the other S. 50° E., indicating at least two ice movements. 
These visitations of the ice must have swept off all previously decom¬ 
posed material from the diabase, so that the present accumulation is in 
some degree a measure of the work of the destructive agencies since the 
second of the two ice invasions to which reference has been made. 
